1. Field of the Invention
The present invention relates to a position detection apparatus used in the assembly process of a factory manufacturing line for visual recognition processing of image data obtained from a target object by an imaging means, and recognizing such primary features of the target object as shape, position, and attitude for use in such processes as pass/fail quality determination and positioning for equipment operation. The present invention is especially suitable for detecting a position of target object under a poor condition for an image recognition method where there is no or poor contrast in the image obtained.
2. Description of the Prior Art
Various position detection principles have been applied according to the application and shape of the target object in conventional position detection apparatuses. A conventional position detection apparatus using the position detection principle is described below with reference to FIG. 13.
In FIG. 13, one example of conventional position detection apparatuses is shown. This position detection apparatus detects the center position of a circular subject. An image pickup device 16 captures an image of objects Ob including first and second object members Ob1 and Ob2 provided with first and second holes H1 and H2 formed therein, respectively. The second object member Ob2 is placed under the first object member Ob1, and is exposed to the image pickup device 16 through the first hole H1. The image pickup device 16 outputs an image density data Sv representing the object image as the difference of density at each of the pixels. The binarization circuit 17 binaries the image data Sv to produce a binary data Sb. The center position detector 18 detects a clustered region indicative of first and second objects Ob1 and Ob2, respectively, based on the binary data Sb, and calculates a center position of the target object Ob2. Based on the thus calculated center position of the target object Ob2, a position of the entire object Ob1 can be detected accurately.
However, when the second hole H2 partially hidden by the first object Ob1, as best shown in FIG. 12, is the target to detect, the center position of the target hole H2 is detected based on the image thereof only shown inside the first hole H1. Therefore, the thus detected center position is not a true center of the target hole H1. Furthermore, where there is no or poor contrast in image brightness between the areas indicating the target hole H2 and the first object Ob1, both areas are clustered into a signal area, disabling to distinguish the target hole H2 and it's background area representing the first object Ob1.